1. Field of the Invention
The present invention relates to semiconductor devices and manufacturing methods thereof, and more particularly, relates to a semiconductor device including a field effect transistor and a diode on the same semiconductor substrate, and also relates to a manufacturing method thereof.
2. Description of the Related Art
As a technique for forming a field effect transistor (FET) and a diode on the same semiconductor substrate, for example, techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 8-340213 and 8-255838 may be considered.
In Japanese Unexamined Patent Application Publication No. 8-340213, a technique has been disclosed in which, on the same semiconductor substrate, a part of one layer is used an active layer for a Schottky diode, and another part of the one layer is used as an active layer for an FET.
In addition, in Japanese Unexamined Patent Application Publication No. 8-255838, the structure has been disclosed in which a PIN diode, an FET, and a heterojunction bipolar transistor (HBT) are formed on the same semiconductor substrate. In this structure, for the PIN diode, the FET, and the HBT, the individual active layers are formed separately.
According to Japanese Unexamined Patent Application Publication No. 8-340213, a contact layer of an anode of a Schottky diode and contact layers of a drain and a source electrode of an FET are formed from the same layer. However, when the contact layers mentioned above are only formed from the same layer, only one step of forming a layer can be omitted in a process for growing active layers. As a result, the effect of reducing the number of manufacturing steps and the effect of reducing cost cannot be fully achieved. In addition, according to Japanese Unexamined Patent Application Publication No. 8-340213, although a manufacturing method of a semiconductor device disclosed therein has not been clearly described, due to variations in degree of etching in an etching step which may be necessary for the manufacturing, variations in properties of the diode and the FET may be increased in some cases.
On the other hand, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-255838, since the active layers of the diode, the FET, and the HBT are individually formed by selective growth, reduction in number of manufacturing steps and reduction in cost cannot be sufficiently achieved.
In addition to the desire of forming the active layer of an FET and that of a diode from a common layer, in order to reduce the number of manufacturing steps, it has been desired that at least one electrode (a gate electrode, a drain electrode, or a source electrode) of the FET and at least one electrode (an anode and a cathode) of the diode are preferably formed at the same time.
When an FET having a Schottky junction for a gate electrode and a diode having a Schottky junction are formed on the same semiconductor substrate, since a layer is provided which is used in common for the active layers thereof, the desire described above can be relatively easily fulfilled.
On the other hand, when an FET, which has a Schottky junction for a gate electrode and has a gate recess, and a pn junction diode are formed on the same semiconductor substrate, the desire described above cannot be so easily fulfilled for the following reasons.
That is, when a drain electrode and a source electrode of the FET are formed on respective contact layers, and a cathode of a diode is simultaneously formed on an n-type layer which is the same type as that of the contact layers, since these electrodes are ohmic electrodes to an n-type layer, by using the same metal, the formation of the electrodes described above can be easily realized. However, metals for a gate electrode, which is a Schottky electrode, and an anode of a diode, which is an ohmic electrode provided on a p-type layer, are formed of a different type of metal from that for the drain electrode and the like, and hence the electrodes described above must be formed in a separate step.
It is not always impossible to simultaneously form a gate electrode, a drain electrode, and a source electrode. However, since a specific step must be additionally required, it is not practical in order to achieve cost reduction. In addition, although both the anode and the cathode are ohmic electrodes in view of functionality, the types of semiconductor layers on which they are to be formed are different from each other, that is, a p-type layer and an n-type layer, different electrode materials must be used, and as a result, the anode and the cathode cannot be simultaneously formed.
According to the situations described above, a desire may arise in that at least a gate electrode and an anode are simultaneously formed. When the gate electrode is formed, a mask is formed for forming a gate recess, and recess etching is then performed. Subsequently, in general, by using the same mask as described above without being removed from the position for the recess forming, a step of forming a gate electrode is performed by deposition or the like.
As described above, the reason that the same mask is used in the etching step of forming the gate recess and in the step of forming the gate electrode by deposition or the like is that by using the same mask, the gate electrode can be formed while the positional accuracy obtained in forming the gate recess is maintained. When a different mask is used, or when once the mask is removed and is then again disposed, the gate electrode cannot be formed with high positional accuracy.
As described above, when the gate electrode and the anode are simultaneously formed using the same mask as that used for forming the gate recess, it is necessary that the mask has an opening at a position at which the anode is to be formed, that is, at which a p-type layer is to be exposed. However, when an opening is provided in the mask for exposing the p-type layer, in an etching step of forming the gate recess, the p-type layer is also etched, and as a result, it disadvantageously becomes difficult to form a diode having desired properties.